Motorcycle tire with tread having main blocks and lower height sub blocks

ABSTRACT

A motorcycle tire is provided that is capable of remarkably improving a traction characteristic and an anti-sideslip characteristic for cornering without deteriorating the mud expulsion characteristic especially on a dirt road. In embodiments, a motorcycle tire has a plurality of main blocks scattered on a tread. Sub blocks are provided between the main blocks. The sub blocks are made of rubber and have a block height H 2  lower than that the height H 1  of the main blocks. A part of the rubber constituting a top portion of the sub block is removed to reduce the area of a upper face of the sub block.

TECHNICAL FIELD

The present invention relates to a motorcycle tire and especially to anoff-road motorcycle tire such as a motocross tire suitably used on anoff-road such as a dirt road.

BACKGROUND ART

A motorcycle tire used on an off-road such as a dirt road generally hasa tread pattern provided with a plurality of blocks. Such a motorcycletire is particularly desired to have a cornering characteristic on adirt road at a higher level because the cornering characteristic as wellas traction and breaking characteristics are considerably importantamong the various required characteristics. Especially, for a sport tireused in a motocross race or the like, a motorcycle turns at such a highrate of speed so that a lateral gripping force of the tire becomes acritically important factor in determining how well the corneringcharacteristic is.

However, a conventional motorcycle tire retro fitted especially for dirtroad conditions has a basic pattern in which a plurality of widthdirection block rows consist of a plurality of blocks located along thetire's width direction and the block rows are arranged along the tire'scircumferential direction with a space therebetween from the viewpointof getting better traction. Further, no block is generally placedbetween the width direction block rows to ensure a good mud expulsioncharacteristic. As a result, the tire has a poor resistance to a sideforce in a middle camber region which is most often used during thecornering on the dirt road. It is thus difficult to balance ananti-sideslip characteristic and a traction characteristic. As usedherein, the middle camber region means a region between 10 and 40degrees provided that a upstanding position of a motorcycle is 0 degreesand its horizontally banking position is 90 degrees.

That is, in the conventional motorcycle tire, if the block pitch in thewidth direction block rows 101 is increased to give a higher negativeratio for an improvement of the mud expulsion characteristic as shown inFIG. 7, the side force resistance is decreased, causing a pooranti-sideslip characteristic. Meanwhile, if a block which has the sameheight as that of the blocks constituting the width direction block row101 is arranged between a width direction block row 101 and an adjacentwidth direction block row 101 for the purpose of enhancing the sideforce resistance and improving the anti-sideslip characteristic, such aproblem occurs that deteriorates the mud expulsion characteristic. It istherefore extremely difficult for the conventional tire to satisfy boththe traction and cornering characteristics without involving adeterioration of the mud expulsion characteristic.

Moreover, it is important for an off-road motorcycle radial tire,especially for the one used in motocross, to assure a rigid sidewall dueto an attribute of its radial configuration. The tire rim will be hitand damaged primarily by a hard road surface if the rigidity of thesidewall is insufficient. Countermeasures such as simply increasing acase rigidity or increasing the inner pressure in use are effective waysto avoid the rim from being damaged. Such countermeasures, however, maycause a reduction in its capability to absorb bumps and gaps on the roadsurface, so that the block rigidity of the tread surface is necessarilylower than that of a bias tire, which decreases the gripping performanceat the same time. Consequently, it has been extremely difficult for anoff-road radial tire to improve its cornering characteristic as well asassure the rigid sidewall without reducing the capability to absorbbumps and the like on the road surface.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide amotorcycle tire capable of remarkably improving a tractioncharacteristic and an anti-sideslip characteristic for cornering withoutdeteriorating the mud expulsion characteristic especially on a dirtroad. It is another object of the present invention to provide amotorcycle pneumatic radial tire which will improve a corneringcharacteristic as well as assure a rigid sidewall while maintaining acapability to absorb bumps on the surface of a dirt road.

To achieve the above-mentioned objects, a motorcycle tire according tothe present invention has a plurality of main blocks scattered on atread and is characterized in that sub blocks are provided between themain blocks, said sub block being made of rubber and having block heightlower than that of the main block, and a part of the rubber constitutinga top portion of the sub block has been removed to reduce the area of aupper face of the sub block.

In such a motorcycle tire, the sub blocks are preferably positionedbetween the main blocks adjacent with each other in the tire's widthdirection, between the main blocks adjacent with each other in thetire's circumferential direction, and/or between the main blocksadjacent with each other in the direction slanted in relation to thetire's circumferential direction.

Further, it is preferable that the negative ratio is between 65% and97%; a tread curvature ratio obtained when a distance measured along thetire's radial direction between a tire sectional height position and atread edge position is divided by the tread width is between 0.20 and0.50; a block area ratio defined as the ratio of total area of upperfaces of top portions of the sub blocks to a total area of upper facesof the main blocks is between 0.05 and 1.2 in each of plural basicpattern elements constituting the tread; and a block height ratiodefined as a ratio of a block height of the sub blocks at its upper faceto a block height of the main blocks is between 0.3 and 0.8. As usedherein, the term “negative ratio” refers to the amount of the space ofthe grooves, calculated as a percentage of the tread area in each of thebasic patter elements, on condition that the upper face area of the topportion of the sub block is included in the groove. Both of the negativeratio and the block area ratio are calculated from the upper face areaof the main block, the upper face area of the top portion of the subblock, and the space of the groove all of which present in each of thebasic patter elements of the developed tread.

Furthermore, it is preferable that the negative ratio is between 75% and97%, the tread curvature ratio is between 0.20 and 0.50, and the blockarea ratio is between 0.2 and 1.2.

Alternatively, it is preferable that the negative ratio is between 65%and 85%, the tread curvature ratio is between 0.20 and 0.50, and theblock area ratio is between 0.05 and 0.5.

It is also preferable that when a bending portion is provided at a midpoint of a side wall extending from a groove bottom toward the upperface of the sub block and the sub block is divided into a bottom portionand a top portion, the upper face area of the top portion is between 0.2and 0.8 times as much as the upper face area of the bottom portion.

The height of said bending portion from the groove bottom along thetire's radial direction is preferably at least 0.5 times and less than1.0 times as much as the block height of the sub block.

The sub block preferably has such a shape that a dimension in the tire'scircumferential direction is greater than a dimension in the tire'swidth direction.

The tire according to the present invention is preferably a radial tire.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a partial development of a tread of one embodiment of atire according to the present invention;

FIG. 1( b) is a sectional view taken along the line I_(A)-I_(A) of FIG.1( a);

FIG. 1( c) is a sectional view taken along the line I_(B)-I_(B) of FIG.1( a);

FIG. 2( a) is a partial development of a tread of another embodiment ofa tire according to the present invention;

FIG. 2( b) is a sectional view taken along the line A-A of FIG. 2( a);

FIG. 2( c) is a sectional view taken along the line B-B of FIG. 2( a);

FIG. 2( d) is a sectional view taken along the line C-C of FIG. 2( a);

FIG. 3( a) is a partial development of a tread of another embodiment ofa tire according to the present invention;

FIG. 3( b) is a sectional view taken along the line A-A of FIG. 3( a);

FIG. 3( c) is a sectional view taken along the line B-B of FIG. 3( a);

FIG. 3( d) is a sectional view taken along the line C-C of FIG. 3( a);

FIG. 4( a) is a partial development of a tread of another embodiment ofa tire according to the present invention;

FIG. 4( b) is a sectional view taken along the line A-A of FIG. 4( a);

FIG. 5( a) is a partial development of a tread of another embodiment ofa tire according to the present invention;

FIG. 5( b) is a sectional view taken along the line A-A of FIG. 5( a);

FIG. 6 shows plan views of sub blocks of various embodiments of tiresaccording to the present invention;

FIG. 7 is a partial development of a tread of a conventional rear tire;

FIG. 8( a) is a partial development of a tread of the tire ofComparative Examples 1 and 2;

FIG. 8( b) is a sectional view taken along the line I_(A)-I_(A) of FIG.8( a);

FIG. 8( c) is a sectional view taken along the line I_(B)-I_(B) of FIG.8( a);

FIG. 9 is a partial development of a tread of the tire of ComparativeExample 3;

FIG. 10( a) is a partial development of a tread of the tire ofComparative Example 4;

FIG. 10( b) is a sectional view taken along the line A-A of FIG. 10( a);

FIG. 10( c) is a sectional view taken along the line B-B of FIG. 10( a);and

FIG. 10( d) is a sectional view taken along the line C-C of FIG. 10( a).

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, illustrative embodiments of the present invention will bedescribed with reference to the drawings. FIG. 1( a) is a partialdevelopment of a tread of a representative embodiment of a tireaccording to the present invention, FIG. 1( b) is a sectional view takenalong the line I_(A)-I_(A) of FIG. 1( a), and FIG. 1( c) is a sectionalview taken along the line I_(B)-I_(B) of FIG. 1( a).

The motorcycle tire (hereinafter referred to as “tire”) shown in FIGS.1( a)-(c) has a plurality of main blocks 2 scattered on a tread 1.

A constitutional feature of the present invention is intended to improvea traction characteristic and an anti-sideslip characteristic especiallyon a dirt road without involving a deterioration of a mud expulsioncharacteristic. More specifically, the constitutional feature is thatsub blocks 3 are provided between the main blocks 2, said sub block 3being made of rubber and having block height H2 lower than the height H1of the main blocks 2, and a part of the rubber constituting a topportion 9 of the sub block 3 has been removed to reduce the area of aupper face 5 of the sub block 3.

In the next, it will be discussed, along with its operation, how theabove-mentioned constitution has been adopted to the present invention.

The present inventor has examined a way of satisfactorily balancing thetraction characteristic and the anti-sideslip characteristic especiallyon a dirt road without involving a deterioration of the mud expulsioncharacteristic. As a result, it is found that the sub block 3 which isprovided between the main blocks 2 on the tread and which has the blockheight H2 lower than the height H1 of the main block 2 can produce aresistance force acting against a part of the mud stream flowing betweenthe main blocks 2. It is also found that the sub block 3, however, doesnot completely block off the mud stream, so that both of the tractioncharacteristic and the anti-sideslip characteristic can be improvedwithout involving a serious deterioration of the mud expulsioncharacteristic.

The present inventor has also experimentally manufactured various typesof tires having different shapes of the sub blocks 3, particularlyshapes of the top portions 9 to examine a relation between the shape ofthe top portion and the above-mentioned characteristics. As a result, itis found that the sub block 3 having an appropriate shape of the topportion, i.e. the sub block 3 having a upper face 5 with its area beingreduced by removing a part of the rubber constituting the top portion 9of the sub block 3 can further enhance the traction characteristic andthe anti-sideslip characteristic. In addition, it is found that such asub block may possibly improve the mud expulsion characteristic. Thepresent invention has been completed on the basis of these findings.

As means for reducing the area of the upper face 5 of the sub block 3,it is preferred to chamfer the edge of the sub block, as shown in FIGS.1( a)-(c), or to form a depression 11 at the central region of the upperface 5, as shown in the B-B section of FIG. 2. With such means, aso-called spike effect which facilitates an intrusion of the top portion9 of the sub block 3 into a dirt road is enhanced, resultingimprovements in both of the traction characteristic and theanti-sideslip characteristic. In the case where the edge of the subblock is chamfered as shown in FIGS. 1( a)-(c), mud can easily pass overthe sub blocks 3 so that the mud expulsion characteristic will beimproved.

In order to emphasize particularly the traction characteristic and thestraight-line stability, the sub block 3 is preferably positionedbetween the main blocks 2 and 2 adjacent with each other in the tire'swidth direction w. In order to emphasize particularly the anti-sideslipcharacteristic, the sub block 3 is preferably positioned between themain blocks 2, 2 adjacent with each other in the tire's circumferentialdirection c. If both of the traction characteristic and theanti-sideslip characteristic should be improved in a well-balancedmanner, the sub block is preferably positioned between the main blocks 2and 2 adjacent with each other in the direction slanted in relation tothe tire's circumferential direction c. If necessary, the sub block 3may be positioned in any two or more spaces existing between theabove-mentioned three sets of the main blocks 2 and 2.

Further, the negative ratio is preferably between 65% and 97%. When theratio is less than 65%, mud tends to clog in the tread to decrease thetraction characteristic. When the ratio is more than 97%, the blockrigidity of the main block 3 is difficult to be maintained.

The tread curvature ratio obtained when a distance measured along thetire's radial direction between a tire sectional height position and atread edge position is divided by the tread width is preferably between0.20 and 0.50. As a motorcycle turns with its body banking, which isso-called a camber running, the tread contact area in the corner willbecome too small and thus the steering stability will decrease when thetread curvature ratio is less than 0.20. To the contrary, the treadcontact area during on the straight line will become too small and thusthe straight-line stability will decrease when the tread curvature ratiois more than 0.50.

The block area ratio defined as the ratio of total area of upper faces 5of the sub blocks 3 to a total area of upper faces 6 of the main blocks2 is preferably between 0.05 and 1.2 in each of plural basic patternelements constituting the tread. When the block area ratio is less than0.05, the amount of the area of the sub blocks 3 is too small and thusthe traction characteristic and the anti-sideslip characteristic cannotbe sufficiently improved. When the block area ratio is more than 1.2,the distance between the sub blocks 3 is too narrow and thus the mudexpulsion characteristic decreases to make mud clogged in the tread.

In addition, the block height ratio defined as a ratio of a block heightH2 of the sub blocks 3 at its upper face 4 to a block height H1 of themain blocks 2 is preferably between 0.3 and 0.8. When the block heightratio is less than 0.3, the height of the sub block 3 is not enough tobe able to sufficiently improve the traction characteristic and theanti-sideslip characteristic. When the block height ratio is more than0.8, the mud expulsion characteristic decreases to make mud clogged inthe tread.

If the tire of the present invention is attached particularly to a frontwheel of a motorcycle, the negative ratio is preferably between 75% and97%, and the block area ratio is preferably between 0.2 and 1.2. Since amotorcycle front tire needs an especially complicated steeringoperation, it is advantageous from the view point of improving steeringstability that the negative ratio and the block area ratio are set tolarge, thereby improving the anti-sideslip characteristic.

If the tire of the present invention is attached to particularly to arear wheel of a motorcycle, the negative ratio is preferably between 65%and 85%, and the block area ratio is preferably between 0.05 and 0.5.Since a motorcycle rear tire needs to effectively transmit the drivingforce from the engine to the road surface, it is advantageous that thenegative ratio and the block area ratio are set to low to improveespecially the traction characteristic and the anti-sideslipcharacteristic.

The ratio of the area of the upper face 5 of the top portion 9 to thearea of the upper face of the bottom portion in the sub block 3 ispreferably between 0.2 and 0.8. When the ratio is less than 0.2, theblock rigidity of the top portion of the sub block 3 is insufficient.When the ratio is more than 0.8, a prompt mud expulsion cannot beexerted.

The ratio of height H3 of the bending portion 8 along the tire's radialdirection to the block height H2 of the sub block 3 is preferablybetween 0.5 and 1.0. When the ratio is less than 0.5, the resistanceforce against the mud stream gets weaker and thus the tractioncharacteristic and the anti-sideslip characteristic may unfavorablydecrease. When the ratio is 1.0, the bending portion 8 agrees with theupper face 5, i.e. the sub block has no bending portion 8, which willnot improve the mud expulsion characteristic at all.

The sub block 3 preferably has such a shape that a dimension L in thetire's circumferential direction is greater than a dimension W in thetire's width direction if the sub block 3 is arranged between the mainblocks 2, 2 adjacent with each other in the tire's circumferentialdirection c as shown in FIG. 1( a). The sub block 3 has such a shapethat the dimension in the tire's circumferential direction c is longerso that a large resistance force against the mud stream flowing in thetire's width direction can be exerted to improve the anti-sideslipcharacteristic. As well, the sub block 3 has such a shape that thedimension in the tire's width direction w is shorter so that the mudstream flowing in the tire's width direction can easily pass over thesub block 3 to assure the mud expulsion characteristic. The tire havingsuch characteristics is suitably used for the rear tire for which ahigher anti-sideslip characteristic is demanded.

When the tire of the present invention is adapted to a motorcyclepneumatic radial tire, a ply material having high modulus can be usedand the side height can be reduced. As a result, the side wall rigiditycan be assured and the cornering characteristic can be improved whileits capability to absorb bumps, gaps and the like on the off-roadsurface is maintained.

Although illustrative embodiments of the present invention have beendescribed above, variations and modifications may be made withoutdeparting from the scope of the invention as defined by the appendedclaims. For example, as shown in FIG. 2, a depression 11 having a heightlower than the surroundings is so formed on the upper face 5 of the subblock 3 that the upper face 5 of the sub block 3 has a doughnut-likeplane shape. Further, the block height H2 of the sub block 3 may differwith respect to each sub block. At least the upper face 5 of the subblock 3 located in the tread central region may be configured todecrease its area, as shown in FIGS. 2 and 3.

Various kinds of shapes as shown in FIG. 6( a) including polygonalshapes such as a quadrangle, round shapes such as an ellipsoid and aperfect circle, shapes formed by a combination of lines and arcs, andshapes formed by arcs, as well as shapes having fabricated depressionson the above-mentioned shapes as shown in FIG. 6( b) may also be adoptedto the planar shape of the sub block 3.

EXAMPLES

Three types of pneumatic tires according to the present invention wereexperimentally manufactured and their performances were evaluated. Thedetails will be described below.

(1) Bias Tire for Rear Wheel

Example 1 was a motorcycle bias tire with a tread pattern shown in FIG.1 and parameters shown in Table 1. For the purpose of a comparison, amotorcycle bias tire with a tread pattern shown in FIG. 8 and parametersshown in Table 1 (Comparative Example 1) was also prepared. Example 1and Comparative Example 1 each had a carcass consisting of two biasplies formed by rubber-coating nylon cords extending at ±30 degrees inrelation to the radial direction.

(2) Radial Tire for Rear Wheel

Example 2 was a motorcycle radial tire with a tread pattern shown inFIG. 1 and parameters shown in Table 2. For the purpose of a comparison,a motorcycle radial tire with a tread pattern shown in FIG. 8 andparameters shown in Table 2 (Comparative Example 2) was also prepared.Example 2 and Comparative Example 2 each had two belt layers made byrubber-coating aramid cords and a belt-reinforcing layer made byrubber-coating nylon cords.

(3) Tire for Front Wheel

Examples 3-6 were motorcycle bias tires with tread pattern shown inFIGS. 2-5, respectively, and parameters shown in Table 3. For thepurpose of comparison, motorcycle bias tires with tread patterns shownin FIGS. 9 and 10, respectively, and parameters shown in Table 3(Comparative Examples 3 and 4) were also prepared. Examples 3-6 as wellas Comparative Examples 3 and 4 had carcasses consisting of two biasplies formed by rubber-coating nylon cords extending at ±30 degrees inrelation to the radial direction.

TABLE 1 (1) Bias tire for rear wheel (Tire size: MCS 110/90-19, Rimsize: 2.15 × 19) Upper face area of top Tread portion/upper faceNegative curvature Block Block area of bottom ratio ratio area ratioheight H2/H1 H3/H2 portion Example 1 81% 0.33 0.05  H1 = 17.5 mm 0.3 0.80.7 H2 = 5.3 mm H3 = 4.2 mm Comparative 81% 0.33 0.06  H1 = 17.5 mm 0.3— — Example 1 H2 = 5.3 mm

TABLE 2 (2) Radial tire for rear wheel (Tire size: MCS 120/90R18, Rimsize: 2.50 × 18) Upper face area of top Tread portion/upper faceNegative curvature Block Block area of bottom ratio ratio area ratioheight H2/H1 H3/H2 portion Example 2 81% 0.32 0.05  H1 = 17.5 mm 0.3 0.80.7 H2 = 5.3 mm H3 = 4.2 mm Comparative 81% 0.32 0.06  H1 = 17.5 mm 0.3— — Example 2 H2 = 5.3 mm

TABLE 3 (3) Bias tire for front wheel (Tire size: MCS 80/100-21, Rimsize: 1.60 × 21) Upper face area of top Tread portion/upper faceNegative curvature Block Block area of bottom ratio ratio area ratioheight H2/H1 H3/H2 portion Example 3 97% 0.45 1   H1 = 15 mm 0.8 0.580.5 H2 = 12 mm H3 = 47 mm Example 4 97% 0.45 1.2 H1 = 15 mm 0.5 0.58 0.6 H2 = 7.5 mm H3 = 7 mm  Example 5 97% 0.45 0.5 H1 = 15 mm 0.8 0.58 0.8H2 = 12 mm H3 = 7 mm  Example 6 97% 0.45 0.5 H1 = 15 mm 0.8 0.58 0.7 H2= 12 mm H3 = 7 mm  Comparative 97% 0.45 — H1 = 15 mm — — — Example 3Comparative 97% 0.45 1.2 H1 = 15 mm 0.8 — — Example 4 H2 = 12 mmTest Procedure

The above-mentioned test tires were equipped on the rims as shown inTable 1-3 to form tire/wheel assemblies. Then, these tire/wheelassemblies were subjected to test ridings under the condition of thetire pressure of 80 kPa and the tire load corresponding to the statewhere one person is on board.

The tires of Examples 1 and 2, Comparative Examples 1 and 2 were mountedon the rear wheels. In this regard, the tire of Comparative Example 3was mounted on the front wheel.

The tires of Examples 3-6, Comparative Examples 3 and 4 were mounted onthe front wheels. In this regard, the tire of Comparative Example 1 wasmounted on the rear wheel.

A professional rider rode motorcycles equipped with the test tires on anoff-road and sensory evaluated the traction characteristic,anti-sideslip characteristic and mud expulsion characteristic. Further,the rigidity and the capability to absorb bumps and gaps were alsoevaluated.

The test results were shown in Tables 4-6, wherein each performance wasrepresented by an index on a scale on which 10 is full marks. The largerthe index value, the better the performance.

TABLE 4 Traction char. Anti-sideslip char. Mud expulsion char. Example 16 7 6 Comparative 5 5 5 Example 1

TABLE 5 Capability to Mud absorb Traction Anti-sideslip expulsion bumpschar char char Rigidity and gaps Example 2 5 7 5 6 6 Comparative 5 5 5 55 Example 2

TABLE 6 Traction char. Anti-sideslip char. Mud expulsion char. Example 38 8 4.5 Example 4 8 8 5 Example 5 6 7 5 Example 6 7 7 5 Comparative 5 55 Example 3 Comparative 6 6 4 Example 4

As seen from the results of Tables 4-6, it is clear that each tire ofExamples has a superior overall performance in terms of the mudexpulsion characteristic, traction characteristic and anti-sideslipcharacteristic as compared with the tires of Comparative Examples.Moreover, the rigidity and the capability to absorb bumps and gaps ofthe radial tire for a rear wheel are improved as compared with the tiresof Comparative Examples.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide amotorcycle tire capable of remarkably improving a tractioncharacteristic and an anti-sideslip characteristic for cornering withoutdeteriorating the mud expulsion characteristic on a dirt road. It isalso possible to provide a motorcycle pneumatic radial tire, especiallythe one used in motocross, which will improve a cornering characteristicas well as assure a rigid sidewall while maintaining a capability toabsorb bumps and gaps on the surface of an off-road.

1. A motorcycle tire, comprising: a plurality of main blocks scatteredon a tread; and sub blocks provided on a centerline of the tire betweenthe main blocks in a tire circumferential direction, each sub blockbeing made of rubber and having a block height lower than that of themain blocks, each sub block comprising: a top portion extending downwardfrom an upper face of the sub block to a bending portion of a sidewallof the sub block; and a bottom portion extending downward from thebending portion to a groove surface, wherein an area of the upper faceof the sub block is less than a cross sectional area of the sub block ata height of the bottom portion, and the bending portion extends around aperimeter of the sub block.
 2. A motorcycle tire, comprising: aplurality of main blocks scattered on a tread; and sub blocks providedon a centerline of the tire between the main blocks in a tirecircumferential direction, each sub block being made of rubber andhaving a block height lower than that of the main blocks, each sub blockcomprising a bending portion that extends around a perimeter of the subblock at a mid point of a side wall extending from a groove bottomtoward an upper face of the sub block, wherein, an upper face area ofthe sub block is smaller than a cross sectional area of the sub block ata height of the bending portion.
 3. A motorcycle tire, comprising: aplurality of main blocks scattered on a tread; and sub blocks providedon a centerline of the tire between the main blocks in a tirecircumferential direction, each sub block being made of rubber andhaving a block height lower than that of the main blocks, each sub blockcomprising: a bottom portion extending radially upward from the treadand having a side that maintains substantially a constant angle withrespect to the radial direction; and a top portion extending radiallyupward from the bottom portion to an upper face of the sub block,wherein a cross sectional area of the upper face of the sub block isless than a cross sectional area of the sub block at a radiallyuppermost portion of the bottom portion based on at least a change inthe angle of the side of the sub block radially upward from the bottomportion, the change in the angle of the side of the sub block extendsaround a perimeter of the sub block.
 4. The motorcycle tire according toclaim 1, further comprising sub blocks positioned between the mainblocks adjacent with each other in a tire width direction.
 5. Themotorcycle tire according to claim 1, further comprising sub blockspositioned between the main blocks adjacent with each other in a tirecircumferential direction.
 6. The motorcycle tire according to claim 1,further comprising sub blocks positioned between the main blocksadjacent with each other in a direction slanted in relation to a tirecircumferential direction.
 7. The motorcycle tire according to claim 1,wherein a negative ratio is between 65% and 97%, the negative ratiobeing an amount of space of a groove, calculated as a percentage of atread area in each of plural basic pattern elements constituting thetread on condition that the upper face area of the top portion of thesub block is included in the groove; a tread curvature ratio, obtainedwhen a distance measured along the a tire radial direction between atire sectional height position and a tread edge position is divided bythe tread width, is between 0.20 and 0.50; a block area ratio, definedas the ratio of total area of upper faces of top portions of the subblocks to a total area of upper faces of the main blocks, is between0.05 and 1.2 in each of plural basic pattern elements constituting thetread; and a block height ratio, defined as a ratio of a block height ofthe sub blocks at an upper face to a block height of the main blocks, isbetween 0.3 and 0.8.
 8. The motorcycle tire according to claim 7,wherein said negative ratio is between 75% and 97%, said tread curvatureratio is between 0.20 and 0.50, and said block area ratio is between 0.2and 1.2.
 9. The motorcycle tire according to claim 7, wherein saidnegative ratio is between 65% and 85%, said tread curvature ratio isbetween 0.20 and 0.50, and said block area ratio is between 0.05 and0.5.
 10. The motorcycle tire according to claim 1, wherein the upperface area of the top portion is between 0.2 and 0.8 times as much as theupper face area of the bottom portion.
 11. The motorcycle tire accordingto claim 1, wherein a bending portion is provided at a mid point of aside wall extending from a groove bottom toward an upper face of the subblock, and the height of said bending portion from the groove bottomalong a tire radial direction is at least 0.5 times and less than 1.0times as much as the block height of the sub block.
 12. The motorcycletire according to claim 1, wherein said sub block has such a shape thata dimension in a tire circumferential direction is greater than adimension in a tire width direction.
 13. The motorcycle tire accordingto claim 1, wherein said tire has a radial carcass.
 14. The motorcycletire according to claim 1, wherein each of the sub blocks is providedbetween a width of a respective main block adjacent to the sub block ina tire circumferential direction.
 15. The motorcycle tire according toclaim 1, wherein the cross sectional area of the upper face of the subblock is less than a cross sectional area of the sub block at a radialuppermost portion of the bottom portion further based on at least adepression in the upper face of the sub block.